This invention relates generally to high voltage cable systems, and in particular to method and apparatus for installing fiber optic cables to be used as distributed temperature sensing transducers for temperature profile measurements in an underground protective duct in which a high voltage cable has already been installed.
The safe working temperature range of cable insulation determines the maximum current loading for most high voltage underground cable systems. Although the thermal performance of cable insulation can be reliably modeled, the thermal parameters of the environments in which cables operate are variable and unpredictable. The environment external to a HV cable transmission system plays a significant role in transferring heat away from cables and is therefore critical to the current capacity rating of cables.
Nearby heat sources, e.g., other cables, road surfaces and other outside plant utility components, will further affect the heat transfer rate and therefore the current rating of the HV cable system. Some unpredictable parameters include: ground ambient temperature, which may change both daily and seasonally, and ground thermal resistivity, that may vary greatly from worst case (dried-out) to best case (wet) conditions.
Generally, the worst-case thermal conditions are not realized in practice and the actual cable current capacity for a particular installation is usually higher than the theoretical design. However, under exceptional circumstances, unforeseen adverse thermal conditions in the cable environment could result in the safe loading being lower than the theoretical design, leading to thermal runaway of the cable and failure of connected loads. Monitoring the temperature profile of the installed HV cable and its environment, plus intelligent processing of the data, can provide early warning of a dangerous operating condition, thus allowing utility operators to take corrective action through up-grading and retrofitting, enabling optimum and safe thermal cable loading to be achieved. For these reasons monitoring of HV cable temperature profiles is of considerable interest to the electric utility operators.
The reality faced by the modem power distribution industry is that equipment more and more is being operated near its maximum current and voltage ratings. Along with these operating conditions comes an increase in unwanted heating of components, including in particular, conductor insulation. The reliability, maintenance and operation life of a high voltage cable are directly affected by its operating temperature. When this temperature exceeds a certain value for any appreciable period of time, it""s useful life rapidly decreases.
Information as to xe2x80x9chot spotsxe2x80x9d and xe2x80x9cover-temperaturexe2x80x9d conditions existing in the HV cable may indicate improper operation, defective components, degradation of insulation, or even possible failure such as short circuit or flash over. Gathering that information, however, is difficult, since the cable is buried underground and operated at very high potentials relative to ground. This high voltage, or its associated electromagnetic interference, hampers measurement of temperature directly on the conductors and makes use of metallic probes ill advised. For one reason, connections or contacts involving metallic conductors or probes are susceptible to dangerous flash-overs. Also, any currents induced in a metallic point temperature sensor (thermocouple) by the high potential could interfere with accurate temperature measurement.
Electric utility companies would like to be able to measure the temperature along the entire length of the HV cable route in order to detect hot spots (areas where the temperature exceeds the safe operating range) that could damage the cable and cause power outages. Optical fibers are now being integrated in most new high voltage cable systems to obtain a distributed temperature profile of the cable serving under load and no-load conditions. Capability of present distributed temperature sensing (DTS) systems allow fiber loop lengths of up to around 12 km for multimode fiber, giving a measurement accuracy of +/xe2x88x9210C. For lengths up to around 30 km, single mode fiber can be used with the same accuracy but with a 3-meter resolution rather than the 1-meter resolution possible with the multimode system. Those systems use a DTS (Distributive Temperature Sensing) unit to send a pulse of laser light through the fiber optic cable and then use a certain light scattering phenomenon that varies with temperature in order to indicate within a few degrees what the temperature is along a specific distribution route.
According to that phenomenon, the intensity ratio between a Stokes line and an anti-Stokes line (which are two components of Raman scattering light) changes sensitively depending on a temperature of an optical fiber. In the measurement, a light pulse is transmitted into the optical fiber, and a time (a delay time until Raman back scattering light returns to a transmitting end of the optical fiber) is measured to determine a position at which the scattering light is generated. The temperature of the optical fiber at that position can be determined by a comparison of the intensity ratio at various points along the sensing cable with the intensity ratio at the sending station where the temperature is known. By detecting the Raman back scattering light from respective positions along the optical fiber on the time division basis, the temperatures at respective positions along the optical fiber, that is, a temperature distribution along the optical fiber can be obtained.
Traditionally, the utility operator would instruct the cable manufacturer to modify the HV cable by installing a small stainless steel tube inside that contains one or more single mode (SM) or multimode (MM) optical fibers. The DTS unit is then connected to the optical fiber and the measurements are taken. There are a number of limitations on this method. The cable is enlarged in diameter to accommodate the steel tube and so is more difficult to install. In addition, the physical pounding that the cables take during installation can also damage the fibers, thus rendering the entire cable out of allowable performance specifications thus resulting in significant expenses to the cable installer, manufacturer and the utility company as well.
Substantial difficulties have been encountered in installing the optical fiber cables in HV ducts in which HV cable has already been installed because of the irregular, high friction surfaces encountered in the duct space. It is therefore desirable to provide improved methods to install optical fiber DTS sensing cable in new HV cable installations as well as during retrofit of older installations, so that the HV cable temperature profile along the entire HV cable run can be monitored.
The present invention provides a cost-effective method to install one or more fiber optic cables in a protective underground duct in which a high voltage cable has already been placed into operation, and so may present variations in the coefficient of friction (COF) on the inside of these ducts that hinder retrofit installation. According to this retrofit method, the fiber cable is not incorporated into the power cable itself, and in some installations does not have direct physical contact with the HV cable.
The insight of the invention is that integration with the HV cable or direct contact of the temperature sensing optical fiber against the HV cable surface is not required for reliable temperature measurements. To the contrary, it is sufficient that the sensing fiber need only be present in the protective duct space, and not in direct contact with the HV cable. This is because the interior space of the protective duct stabilizes at or very near the local HV cable temperature. Even if the sensing fiber is enclosed in a guide tube, upon thermal stabilization, it will nevertheless still accurately sense the HV cable temperature by heat transfer through the guide tube sidewall.
The sensing fiber cable is installed externally to (along side) the HV power cable, either in direct surface contact with the HV cable, or alternatively, the fiber optic cable is installed in a small diameter guide tube that is located in the protective duct space between the HV cable and the protective duct. According to one aspect of the invention, the sensing fiber and one or more guide tubes are installed in a loose bundle at least in part by fluid drag forces (pushing and blowing with pressurized air) using conventional cable launching equipment.
Prior to installation of the optical fibers and small guide tubes, a pair of large diameter guide tubes is placed in the duct annulus by pulling the guide tubes with a rope. The large diameter guide tubes are placed on opposite sides of the HV cable in a wedging position that blocks downward shifting movement of the small guide tubes and optical fibers into the lower cusp-shaped annulus.
This blocking action is needed because otherwise the optical fibers would be pinched or crushed by flexing movement of the HV cable as it reacts to electromagnetic forces caused by rapid changes in the electromagnetic field surrounding the cable, for example while undergoing rapid changes in operating voltage incidental to start-up or sudden load fluctuations. The large diameter guide tubes, in combination with the top surface of the HV cable and the inner bore of the protective duct, form a low friction longitudinal passage for receiving and guiding the optical sensing fibers and guide tubes as they are blown through the protective duct by pressurized air.
After the optical fiber has been installed, DTS analytical software is used to perform the Raman back scattering analysis to give the desired temperature profile analysis. This saves the utility company and cable manufacturer a significant amount of direct costs as well as risk cost.
These optical fibers are pig-tailed to allow the DTS unit to be directly plugged into the fiber at manhole inspection stations along the route. From that point, the data is sent by SCADA or wireless transmission directly to controllers monitoring the system.